You pocket the Watmel Berry the Staravia had and carry on. After Staravia is knocked out, you find a Coba Berry and a Cherish Ball as well. It seems that luck is on your side. With your quarry defeated, you find a path at the very end of the open area that leads down below to the inset lake. Some bushes are growing outside the entrance, and you find one Oran Berry and one Leppa Berry . Highly convenient.

"I see you made it through your trials. Now you'll get a chance to witness something truly legendary..."

The voice weakens and fades, and you are left facing a grotto whose light source appears to come from below the water. You imagine it to be some kind of crystalline structure, since the water is so clear you can see straight to the bottom. Looking upward, you can see the center of the Grotto is directly under the inset lake you saw earlier, and water is flowing down an elevated platform that spans the entire room, covering all the walls with water. But you don't get much time to enjoy the view, as soon enough a Golduck sticks its head out of the water. It dives back under, and soon in the center of the grotto you see a Phione rise upward, floating above the water's surface, and then over a small island in the center of the Grotto. Golduck joins it there. Both seem to be awaiting your actions.